Smart antenna technology is an important technology in modem mobile communications technology, especially in cellular mobile communications systems. Advantages of smart antenna technology include: increased system capacity, increased coverage area of a wireless base station, decreased system cost and greater system performance. Therefore, smart antenna technology has become an important research subject of high technology fields around the world.
A smart antenna system generally comprises: an antenna array having N antenna elements, N radio frequency transceivers and N feeder cables connecting the N antenna elements and the N radio frequency transceivers, respectively. Among them, the N antenna elements and the N feeder cables compose an antenna feeder cable unit. The antenna array and the N radio frequency transceivers compose a radio frequency unit. In a wireless base station, analog signals, transmitted and received by radio frequency units, are transformed by high speed ADC/DAC, and then signals transformed are connected with a data bus, which is connected with a baseband digital signal processor (DSP). Smart antenna functions, such as uplink beam forming and downlink beam forming, are implemented in the baseband DSP.
FIG. 1 shows a wireless base station structure with smart antenna, illustrating the basic structure and working principle of a modern smart antenna. The base station works at CDMA TDD (Code Division Multiple Access, Time Division Duplex). The antenna feeder cable units comprise N antenna elements 11, 12, 13, . . . , 1N, which consist an antenna array, and corresponding feeder cables. Each antenna feeder cable unit is connected with a radio frequency transceiver TRX 21, 22, 23, . . . , 2N. N radio frequency transceivers commonly use one frequency and timing unit 30 (local oscillator), so the radio frequency transceivers 21, 22, 23, . . . , 2N work coherently. Signals received by each radio frequency transceiver are converted to digital sampling signals by an internal ADC of radio frequency transceiver, and then are sent to baseband digital signal processor 33 through high speed data bus 31. Digital signals to be transmitted on high data bus 31 are converted to analog signals by an internal DAC of radio frequency transceiver, and are transmitted by antenna elements 11, 12, 13, . . . , 1N.
All baseband digital signal processing is performed in the baseband digital signal processor 33. Such a processing method is detailed in Chinese Patent No. CN 97104039, the contents of which are incorporated herein by reference. In the baseband processor hardware platform with advanced digital signal processing, processing functions such as modulation and demodulation, receiving and transmitting (uplink and downlink) and beam forming, among others, can be implemented. With these processing functions multiple access interference and multiple path interference can be overcome, and receiving signal-to-noise ratio and sensitivity are raised and EIRP (Equivalent Isotropically Radiated Power) is increased. At present, all smart antennas use a ring antenna array or a linear antenna array, and the ring or linear antenna array is concentrated on one place in order to obtain an isotropical covering or a sector covering, such as disclosed in Chinese Patent No. CN 97104039. In accompanying with increase of dense and high of buildings in city, the working frequency of mobile communication system is relatively high (1 to 3 GHz) in a building or a cell. In this case, due to the shielding function of buildings and loses due to floors and walls, many shaded areas appear and the coverage range of a mobile communication system is limited. Typically, in order to solve the coverage problem, when designing cellular mobile communication system in an urban area of a city, the number of base stations must be increased. However, this solution will increase system investment and maintenance difficulties. Although in theory a smart antenna will improve the coverage range of a base station, if multiple antenna units of an antenna array are concentrated, the coverage problem cannot be fully solved.